Well, first 50 M Baht free then 10%.
If he lives in Thailand and makes a Thai last will for all assets the Thai beneficiary pays only Thai tax even for assets abroad. At least a point to check with an expert.
If different beneficiaries, meaning apart from a Thai, you are right and two wills are the option. But even for that case an expert needs or should be appointed because of some possible interfaces between the wills.
Many people with assets dont know tax optimized last wills. Thats my conclusion after dealing with this matter for years:)